When incomplete combustion of an air-fuel mixture or the like arises within a combustion chamber of an engine, carbon is generated and may accumulate on the surface of an insulator of a spark plug. When the surface of the insulator is covered with carbon, leakage current is generated, and discharge may fail to be generated normally between electrodes (across a spark gap).
A conventionally known technique for restraining leakage current in a spark plug is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-183177.
According to this technique, a portion (hereinafter may be referred to as a “leg portion”) of the insulator of the spark plug which is exposed within the combustion chamber is increased in length. This practice increases the surface area of the leg portion; thus, even when carbon adheres to the leg portion, leakage current is unlikely to be generated, thereby improving fouling resistance of the spark plug. Although this technique can improve fouling resistance, it involves a problem in that, since heat fails to smoothly transfer from the insulator to a metallic member, heat resistance of the spark plug deteriorates.
The present invention has been conceived to solve the above-mentioned conventional problem, and an object of the invention is to provide a technique for restraining the generation of leakage current while maintaining heat resistance of a spark plug.